Method for generating csamt full-zone apparent resistivity-depth profile

Abstract

A method for generating a CSAMT full-zone apparent resistivity-depth profile, comprising: using the horizontal positions [Equation 1] and the vertical positions [Equation 2] of recording points (5) as assignment points for apparent resistivity: in a far zone, a recording point (5) being located at a detection depth (4) right below an observation point (2); in a middle zone, the radius of a wavefront being reduced linearly as an induction number decreases, and a recording point (5) being located at a point of intersection of the wavefront and a straight line from the detection depth (4) to a source point (1), and approaching the source point (1) from an observation point (2) as the induction number decreases; in a near zone, as the induction number continuously decreases, the horizontal position of a recording point (5) approaching a midpoint of an offset (3) from the vicinity of the source point (1), the vertical position thereof approaching a certain constant value proportional to the offset (3) from the vicinity below the source point (1), and the constant value being equal to the offset (3) divided by critical values of the far zone and the middle zone. In order to separate planar and non-planar waves, and eliminate and utilize source overprint and shadow effects, the method is provided, thus improving the interpretation effect of the apparent resistivity-depth profile.

Description

A method for generating CSAMT full-area apparent resistivity-depth profiles Technical Field

[0001] The invention belongs to the field of electromagnetic exploration, and in particular relates to a method for generating a CSAMT full-area apparent resistivity-depth profile. Background Art

[0002] Conventional CSAMT (controlled source audio-frequency magneto-telluric) surveys often include information from the far, middle, and near zones (Zonge and Hughes, 1991, p. 750). As the recording point moves from the observation point toward the source, the proportions of plane and non-plane waves change as the distance from the far zone moves toward the near zone. This creates source duplication and shadowing effects (Zonge and Hughes, 1991, pp. 769–784), which can complicate the interpretation of apparent resistivity profiles.

[0003] For example, existing patent CN113960684A discloses a method for generating apparent resistivity-depth profiles for short-offset electromagnetic surveys. This method provides an algorithm for moving recording points along with induction numbers within the interval from the measurement point to the offset midpoint, improving the adaptability of the apparent resistivity-depth profile interpretation method. However, this method omits the area between the offset midpoint and the source point, and is unable to address the field source copy effect and the shadow effect between the source point and the offset midpoint.

[0004] Summary of the Invention

[0005] To address the deficiencies in the prior art, the present invention proposes a method for generating a CSAMT full-area apparent resistivity-depth profile. The method moves the recording point from the observation point to the vicinity of the source and adopts the full-area field expression (Spies and Frischknecht, 1991, p. 294) to generate a full-area apparent resistivity-depth profile. This method aims to eliminate and utilize the field source and shadow effects, providing a new, simple, and rapid method for separating plane and non-plane waves, thereby improving the interpretation of the apparent resistivity-depth profile.

[0006] The technical solution adopted in the present invention is:

[0007] A method for generating the apparent resistivity-depth profile of the entire CSAMT area, based on the horizontal position of the recording point and vertical position As the value point of apparent resistivity, in the far zone, the recording point is located at the detection depth directly below the measuring point; in the middle zone, the radius of the wavefront decreases linearly with the decrease of the induction number, and the recording point is located at the intersection of the wavefront and the straight line from the detection depth to the source point, and approaches the source point from the measuring point as the induction number decreases; in the near zone, as the induction number continues to decrease, the horizontal position of the recording point approaches the midpoint of the offset from the vicinity of the source point, and the vertical position approaches the constant value from the vicinity below the source point:

[0008] in, is the induction number, r i is the offset detection depth, is the detection depth, i=1,2,…,m is the measurement point number, j=1,2,…,n is the frequency point number, cri far-mid is the critical value between the far zone and the middle zone, cri mid-near is the critical value between the middle zone and the near zone;

[0009] ① Axial device, assume that the source point coincides with the origin O of the rectangular coordinate system, and the measuring line is arranged along the x-axis. If the superscript r is replaced by x, then the horizontal position of each recording point on the xOz plane in the direction of the survey line is recorded as The vertical position of the recording point is recorded as The position of this recording point is the apparent resistivity of each measuring point of the axial device on the xOz plane. The assignment point of 1 axial device measuring line generates 1 apparent resistivity-depth profile along the measuring line direction;

[0010] ② Equatorial device: an equatorial device with m measuring points will generate a profile along the measuring line and m profiles along the source point to the measuring point. For this purpose, the midpoint of the measuring line is set as the coordinate origin O′, the source point coincides with the origin O of the cylindrical coordinate system, the connecting line OO′ is perpendicular to the measuring line, and the direction of the measuring line is set as the x′ axis of the rectangular coordinate system. If the superscript r is replaced by x′, then on the x′O′z plane along the survey line, the recording point of each measuring point is recorded as the horizontal position The vertical position of the recording point is recorded as The location of this recording point is the apparent resistivity of each measuring point of the equatorial device on the x′O′z plane. The assignment point;

[0011] The horizontal position of the recording point of each measuring point on each rOz plane along the section from the source point to the measuring point and vertical position Still using formulas 1a, 1b, and 1c, the location of this recording point is the apparent resistivity of each measuring point of the equatorial device on the rOz plane. The assignment point.

[0012] Furthermore, the fixed value is proportional to the offset distance, which can be expressed as: the fixed value is equal to the offset distance r i Divide by the critical value cri of the far zone and the middle zone far-mid .

[0013] Furthermore, the offset distance r in the equator is i and the measuring point position x i The relationship between ′ is:

[0014] The survey line is arranged along the x' axis of the rectangular coordinate system, and the midpoint of the survey line is taken as the coordinate origin O'. The source point coincides with the origin O of the cylindrical coordinate system, and the connecting line is OO', x' i is the value of the measuring point on the x′ axis.

[0015] Furthermore, the apparent resistivity The electric field must be defined and calculated by the full-area expression of the single-component electric or magnetic field. The overall expression of the component and the simple iterative formula of the overall apparent resistivity are:

[0016] electric field The full-area expression and direct calculation formula of the full-area apparent resistivity are:

[0017] Where, I j is the source current intensity at the jth frequency point, l is the length of the source, j is the imaginary unit, is the angle between the measuring point i and the source, where f i,j is the frequency of the jth frequency point at measuring point i, and μ0 is the magnetic permeability of the non-magnetic earth.

[0018] Further, the depth of detection It can be calculated by the skin depth formula, which is expressed as:

[0019] Among them, f i,j is the frequency of the jth frequency point of measurement point i, is the apparent resistivity of the j-th frequency point at measuring point i.

[0020] Furthermore, the offset is at least cri far-mid times.

[0021] Furthermore, in the axial device, the field critical value cri far-mid and cri mid-near Select 5 and 1 respectively.

[0022] Furthermore, in the equatorial device, the field critical value cri far-mid and cri mid-near Select 4 and 1 respectively.

[0023] Furthermore, the critical value cri of the field area is adjusted according to the geological structure, device, source or observation component. far-mid and cri mid-near .

[0024] Furthermore, in addition to the location of the measuring point and the scale of the source, the location of the field source should also be recorded in the field observation record in order to determine the offset distance. Beneficial effects

[0025] 1) The apparent resistivity-depth profile generated by the method of the present invention is applicable to the entire CSAMT exploration area.

[0026] 2) The present invention provides a new, simple and fast method for separating plane and non-plane waves in CSAMT exploration.

[0027] 3) The present invention extends the movement range of the recording point from the observation point to the source, providing a new simple and fast method to eliminate and utilize the field source copy and shadow effects in CSAMT exploration, thereby improving the interpretation effect of the apparent resistivity-depth profile. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a schematic diagram of the movement of the recording point of a single measuring point along with the induction number.

[0029] FIG2 is a schematic diagram of an axial device and an apparent resistivity-depth profile, wherein FIG2a is a planar layout diagram of the device, and FIG2b is an apparent resistivity-depth profile of the device on the xOz plane.

[0030] Figure 3 is a schematic diagram of the equatorial device and the apparent resistivity-depth profile, where 3a is the planar layout of the device, 3b is the apparent resistivity-depth profile of the device on the x′O′z plane, and Figure 3c is the apparent resistivity-depth profile of the device on the rOz plane.

[0031] FIG4 is a plan view of the axial device of the first embodiment.

[0032] FIG5 is a cross-sectional diagram of apparent resistivity-depth of the axial device of Example 1 along the measuring line direction.

[0033] FIG6 is a plan view of the equatorial device layout of the second embodiment.

[0034] Figure 7 is an apparent resistivity-depth profile of the equatorial device of Example 2, where 7a is the apparent resistivity-depth profile along the measuring line, and 7b, 7c, 7d, 7e and 7f are the apparent resistivity-depth profiles along the lines connecting measuring points No. 1, No. 2, No. 3, No. 4 and No. 5 to the source point, respectively.

[0035] In the figure: 1 source point; 2 measuring point; 3 offset distance; 4 detection depth 5 recording points or 6 point spacing; 7 equal apparent resistivity curves. DETAILED DESCRIPTION

[0036] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0037] The method for generating a CSATM full-area apparent resistivity-depth profile proposed in the present invention specifically includes the following steps:

[0038] To record the horizontal position of the point and vertical position As the assignment point of apparent resistivity (Figure 1): in the far zone, the recording point is located at the detection depth directly below the measuring point; in the middle zone, the radius of the wavefront decreases linearly with the decrease of the induction number, and the recording point is located at the intersection of the wavefront and the straight line from the detection depth to the source point and approaches the source point from the measuring point as the induction number decreases; in the near zone, as the induction number continues to decrease, the horizontal position of the recording point approaches from the vicinity of the source point to the midpoint of the offset distance, and the vertical position of the recording point approaches from the vicinity below the source point to a certain fixed value proportional to the offset distance, which is equal to the offset distance r in this invention. i Divide by the critical value cri of the far zone and the middle zone far-mid ;

[0039] More specifically, the horizontal position of the recording point mentioned above and vertical position The far zone, middle zone and near zone can be expressed as follows:

[0040] Where, is the induction number, r i is the offset detection depth, is the detection depth, i=1,2,…,m is the measurement point number, j=1,2,…,n is the frequency point number, cri far-mid is the critical value between the far zone and the middle zone, cri mid-nearIt is the critical value between the middle zone and the near zone.

[0041] ① Axial device, assuming that the source point coincides with the origin O of the rectangular coordinate system and the measuring line is arranged along the x-axis (Figure 2a). In this embodiment, the field critical values ​​of the axial device are cri far-mid =5 and cri mid-near =1, and the formula (1) If the superscript r is replaced by x, then on the xOz plane (Fig. 2b), the horizontal position of the recording point of each measuring point is and vertical position for:

[0042] The above recording point positions are the apparent resistivity of each measuring point of the axial device on the xOz plane. The assignment point of 1 axial device measuring line generates 1 apparent resistivity-depth profile along the measuring line direction;

[0043] ② Equatorial device: An equatorial device with m measuring points will generate one profile along the measuring line and m profiles along the source point to the measuring point. For this purpose, the measuring line is arranged along the x′ axis of the rectangular coordinate system, the midpoint of the measuring line is taken as the coordinate origin O′, the source point coincides with the origin O of the cylindrical coordinate system, and the connecting line OO′ is perpendicular to the measuring line (Figure 3a). Then the offset distance r in the equatorial device is i and the measuring point position x′ i The relationship is:

[0044] In formula (1a), Replace the superscript r with x′ and take cri far-mid =4, then the horizontal position of each recording point on the x′O′z plane (Figure 3b) and vertical position for:

[0045] The above recording point positions are the apparent resistivity of each measuring point of the equatorial device on the x′O′z plane. The assignment point;

[0046] In this embodiment, the critical values ​​of the equatorial device are cri far-mid =4 and cri mid-near = 1, then the horizontal position of each recording point on the rOz plane (Figure 3c) along the section from the source point to the measuring point is and vertical position for:

[0047] The above recording point positions are the apparent resistivity of each measuring point of the equatorial device on the rOz plane. The assignment point;

[0048] Apparent resistivity The electric field must be defined and calculated by the full-area expression of the single-component electric or magnetic field. The overall expression of the component and the simple iterative formula of the overall apparent resistivity are:

[0049] electric field The full-area expression and direct calculation formula of the full-area apparent resistivity are:

[0050] Where, I j is the source current intensity at the jth frequency point, l is the length of the source, j is the imaginary unit, is the angle between the measuring point i and the source, where f i,j is the frequency of the jth frequency point at measuring point i, and μ0 is the magnetic permeability of the non-magnetic earth.

[0051] Detection depth The skin depth formula can be used

[0052] calculate.

[0053] The following two specific examples are used to further illustrate this:

[0054] Example 1: Generation of a full-area apparent resistivity-depth profile for an axial device. Figure 4 is a plan view of the device, with a total of nine measuring points. Table 1 shows the geoelectrical model on the left and the offset distances of each measuring point on the right.

[0055] Table 1 Offset distances between geoelectric model and each measuring point of axial device

[0056] The first column of Table 2 is the frequency point number, the second column is the operating frequency of each measuring point, and the third column is the apparent resistivity calculated by formula (6b) in this implementation method (the following formulas are all formulas in this implementation method) The 4th column is Substitute the detection depth calculated by formula (8) into The 5th column is the induction number The offset distance r i and detection depth Substituting into formula (2), we can get the horizontal position of each measuring point on the xOz plane: and vertical position Listed in columns 6 and 7 respectively, as the apparent resistivity in column 3 The assignment point.

[0057] Table 2 As Column A, As Column B, As Column C in Table 3, it constitutes a line of apparent resistivity-depth profile data along the survey line. Figure 5 is the apparent resistivity-depth profile of the entire area drawn using Surfer software.

[0058] Table 2 Operating frequency, apparent resistivity, detection depth, induction number and recording points of each measuring point of the axial device

[0059] Recording points of each frequency at measurement point No. 2 (r2 = 800m)

[0060] Recording points of each frequency at measurement point No.3 (r3=1000m)

[0061] Recording points of each frequency at measurement point No. 4 (r4 = 1200m)

[0062] Recording points of each frequency at measurement point No.5 (r5=1400m)

[0063] Recording points of each frequency at measurement point No. 6 (r6 = 1600m)

[0064] Recording points of each frequency at measurement point No.7 (r7=1800m)

[0065] Recording points of each frequency at measurement point No. 8 (r8 = 2000m)

[0066] Recording points of each frequency at measurement point No. 9 (r9 = 2200m)

[0067] Table 3 Data for plotting the full-area apparent resistivity-depth profile of the axial device

[0068] Example 2: Generation of an equatorial device-wide apparent resistivity-depth profile. Figure 6 shows the layout of the device. The survey line is 2000 m from the source point, and the distance between the survey points is 200 m. There are nine survey points in total. The left side of Table 4 shows the geoelectrical model, and the right side shows the offset distances of each survey point calculated using Equation (3).

[0069] Table 4. Offset distances between the geoelectric model and each measuring point of the equatorial device

[0070] The first column of Table 5 is the frequency point number, the second column is the operating frequency of each measuring point, and the third column is the apparent resistivity calculated by formula (6b) The 4th column is Substitute the detection depth calculated by formula (8) into The 5th column is the induction number The offset distance r i and detection depth Substituting into formula (5), the horizontal position of each measuring point on the rOz plane is obtained and vertical position Listed in columns 6 and 7 respectively, as the apparent resistivity in column 3 The assignment point of each measuring point; the horizontal position of the recording point on the x′O′z plane and vertical position It can be directly obtained from the table according to formula (4) as the apparent resistivity in the third column The corresponding assignment point on this plane.

[0071] Table 5 (Including ) as Column A, As Column B, The data are listed as Column C in Table 6, which consists of one apparent resistivity-depth profile along the survey line and nine apparent resistivity-depth profiles from the survey point to the source. Figure 7 is the apparent resistivity-depth profile of the entire area drawn using Surfer software, where Figure 7a is the profile along the survey line, and Figures 7b to 7f are the profiles from the survey point to the source (due to symmetry, only five of the profiles are drawn).

[0072] Table 5 Operating frequency, apparent resistivity, detection depth, induction number and recording points of each measuring point of the equatorial device

[0073] Recording points of each frequency at measuring point No. 2 (r2 = 2088m, x′2 = -600m)

[0074] Recording points of each frequency at measuring point No. 3 (r3 = 2040m, x′3 = -400m)

[0075] Recording points of each frequency at measurement point No. 4 (r4 = 2010m, x′4 = -200m)

[0076] Recording points of each frequency at measuring point No.5 (r5=2000m, x′5=0m)

[0077] Recording points of each frequency at measuring point No. 6 (r6 = 2010m, x′6 = 200m)

[0078] Recording points of each frequency at measuring point No.7 (r7=2040m, x′7=400m)

[0079] Recording points of each frequency at measurement point No. 8 (r8 = 2088m, x'8 = 600m)

[0080] Recording points of each frequency at measuring point No. 9 (r9 = 2154m, x'9 = 800m)

[0081] Table 6 Data for drawing the apparent resistivity-depth profile of the entire equatorial device area

[0082] In addition, the offset distance of the axial device is at least 5 times the total thickness of the detected stratum, the offset distance of the equatorial device is at least 4 times the total thickness of the detected stratum, and the detection depth is It can also be obtained by other improved algorithms.

[0083] The above method is applicable to other types of CSAMT field sources and devices, provided that the definition of apparent resistivity used should still be related to the earth resistivity in the near area.

[0084] The critical value cri of the field region far-mid and cri mid-near Adjustments can be made based on geological structure, device, source or observation components.

[0085] In field observation records, in addition to the location of the measuring point and the scale of the source, the location of the field source should also be recorded in order to determine the offset distance.

[0086] In summary, the present invention provides a method for generating a full-area CSAMT apparent resistivity-depth profile. This method extends the range of recording point movement with induction number from the measurement point to the vicinity of the source point, and provides algorithm formulas for recording points in the far, middle, and near zones. This provides a new, simple and fast method for analytically separating plane and non-plane waves and eliminating and utilizing field source copying and shadowing effects.

[0087] The above embodiments are intended only to illustrate the design concepts and features of the present invention. Their purpose is to enable those skilled in the art to understand the contents of the present invention and implement them accordingly. The scope of protection of the present invention is not limited to the above embodiments. Therefore, any equivalent changes or modifications made based on the principles and design concepts disclosed in the present invention are within the scope of protection of the present invention.

Claims

1. A method for generating a CSAMT full-area apparent resistivity-depth profile, characterized in that: To record the horizontal position of the point and vertical position As the value point of apparent resistivity, in the far zone, the recording point is located at the detection depth directly below the measuring point; in the middle zone, the radius of the wavefront decreases linearly with the decrease of the induction number, and the recording point is located at the intersection of the wavefront and the straight line from the detection depth to the source point and approaches the source point from the measuring point as the induction number decreases; in the near zone, as the induction number continues to decrease, the horizontal position of the recording point approaches from the vicinity of the source point to the midpoint of the offset, and the vertical position approaches from the vicinity below the source point to the fixed value: in, is the induction number, r i is the offset detection depth, is the detection depth, i=1,2,…,m is the measurement point number, j=1,2,…,n is the frequency point number, cri far-mid is the critical value between the far zone and the middle zone, cri mid-near is the critical value between the middle zone and the near zone; ① Axial device, assume that the source point coincides with the origin O of the rectangular coordinate system, and the measuring line is arranged along the x-axis. If the superscript r is replaced by x, then the horizontal position of each recording point on the xOz plane in the direction of the survey line is recorded as The vertical position of the recording point is recorded as This position is the apparent resistivity of each measuring point of the axial device on the xOz plane. The assignment points; 1 survey line of the axial device, generating 1 apparent resistivity-depth profile along the survey line direction; ② Equatorial device: An equatorial device with m measuring points will generate a profile along the measuring line and m profiles from the source point to the measuring point. For this purpose, the midpoint of the measuring line is set as the coordinate origin O′, the source point coincides with the origin O of the cylindrical coordinate system, the connecting line OO′ is perpendicular to the measuring line, and the direction of the measuring line is set as the x′ axis of the rectangular coordinate system. If the superscript r is replaced by x′, then the horizontal position of each recording point on the x′O′z plane along the measuring line is recorded as Record point vertical The position is recorded as The location of this recording point is the apparent resistivity of each measuring point of the equatorial device on the x′O′z plane. The assignment point of The horizontal position of the recording point of each measuring point on each rOz plane along each section from the source point to the measuring point and vertical position Still as shown in formulas 1a, 1b, and 1c, the location of this recording point is the apparent resistivity of each measuring point of the equatorial device on the rOz plane. The assignment point.

2. A method for generating a CSAMT full-area apparent resistivity-depth profile according to claim 1, characterized in that: The fixed value is proportional to the offset distance, expressed as: the fixed value is equal to the offset distance r i Divide by the critical value of the far zone and the middle zone cri far-mid .

3. The method for generating a CSAMT full-area apparent resistivity-depth profile according to claim 1, characterized in that: Equatorial device offset r i and the measuring point position x′ i The relationship is: The survey line is arranged along the x' axis of the rectangular coordinate system, and the midpoint of the survey line is taken as the coordinate origin O'. The source point coincides with the origin O of the cylindrical coordinate system, and the connecting line is OO', x' i is the value of the measuring point on the x′ axis.

4. The method for generating a CSAMT full-area apparent resistivity-depth profile according to claim 1, characterized in that: Apparent resistivity It must be defined and calculated by the full-area expression of the single-component electric or magnetic field. The whole area expression of the component and the simple iterative formula of the whole area apparent resistivity are: electric field The full-area expression and direct calculation formula of the full-area apparent resistivity are: In the formula, I j is the source current intensity at the jth frequency point, l is the length of the source, j is the imaginary unit, is the distance between the measuring point i and the source The angle between where f i,j is the frequency of the jth frequency point at measuring point i, and μ0 is the magnetic permeability of the non-magnetic earth.

5. The method for generating a CSAMT full-area apparent resistivity-depth profile according to claim 1, characterized in that: Detection depth It can be calculated by the skin depth formula, expressed as: Among them, f i,j is the frequency of the jth frequency point at measurement point i, is the apparent resistivity of the jth frequency point at measuring point i; 6. A method for generating a CSAMT full-area apparent resistivity-depth profile according to claim 1 or 2, characterized in that: The offset distance is at least cri of the total thickness of the detected stratum far-mid times.

7. The method for generating a CSAMT full-area apparent resistivity-depth profile according to claim 1, characterized in that: In the axial device, the field critical value cri far-mid andcri mid-near Select 5 and 1 respectively.

8. The method for generating a CSAMT full-area apparent resistivity-depth profile according to claim 1, characterized in that: In the equatorial device, the field critical value cri far-mid andcri mid-near Select 4 and 1 respectively.

9. The method for generating a CSAMT full-area apparent resistivity-depth profile according to claim 1, characterized in that: Adjust the critical value cri of the field area according to the geological structure, device, source or observation component far-mid andcri mid-near .

10. The method for generating a CSAMT full-area apparent resistivity-depth profile according to claim 1, characterized in that: In field observation records, in addition to the location of the measuring point and the scale of the source, the location of the field source should also be recorded in order to determine the offset distance.

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